1. Field of the Invention
The present invention relates to an ignition system primarily for a gasoline engine and more particularly to an ignition system where a discharge bulb for a series gap is built in.
2. Prior Art
An ignition system for a gasoline engine where a high voltage is applied to the ignition plug to produce a spark, may suffer from the problem that deteriorated ignition performance due to current leakage can cause a failure in starting the engine if the ignition plug has poor insulation characteristic due to incomplete combustion or other causes. Also, combustion and ignition require higher discharge voltages and spark energy at lower temperatures below zero than at normal temperatures. Thus, it has been a simple and effective way to overcome these difficulties that high voltages are applied to an ignition plug via a series gap. In order to properly control ignition timing for effective use of series gap, it is necessary that the threshold discharge voltage of series gap is selected to be sufficiently high as compared to that of ignition plug. The use of series gap imposes severe requirements of sustaining voltages of ignition coils and high voltage cords.
Published Japanese Patent Application No. 51-32180 discloses one such ignition plug provided with a built-in discharge bulb for a series gap. This plug is an ignition plug shown in FIG. 5 where opposed electrodes extend into a discharge bulb having an inert gas charged therein. A center electrode 2 is coupled at its rear end to one 8 of the electrodes via a wire 10 while a high voltage terminal 5 is coupled at its front end to the other 7 electrode via a wire 9.
Such conventional built-in series-gap type ignition plug has electrodes that are under repeated discharge at all times during operation of an engine. The series gap plays its important role when the engine is started and for a subsequent short time period of about few minutes or little more than then minutes until the engine is warm enough. The series gap is not necessary once the engine reaches its steady running state. With the conventional ignition system, the series gap remains under repeated discharge during steady operation of engine as well. This not only causes wear and tear of electrodes leading to a shortened life of the discharge bulb but also imposes severe requirements of high-voltage sustaining characteristic on the ignition coil and high voltage cords.